1. Field of the Invention
The invention relates to a high pressure discharge lamp filled with xenon gas as the emitter, which is used as a light source, for example, in a projection apparatus or the like using DLP® (digital light processing) technology.
2. Description of Related Art
Conventionally, a high pressure discharge lamp is one with the arrangement shown, for example, in FIG. 8. This high pressure discharge lamp 10 consists of a bulb of silica glass which has an arc tube 11, and sealing parts 12, with a cathode 19 and an anode 14 arranged opposite each other in the arc tube 11.
Furthermore, tungsten electrode rods 191, 141, which support the cathode 19 and anode 14, respectively, are inserted into a silica glass cylindrical retaining body 16, and thus, retained. Each retaining body 16 is permanently located in a respective one of the sealing parts 12 which has an axial through opening through which the electrode rod extends in the axial direction. Moreover, the electrode rods are sealed in the sealing part 12 by graded glass 15. The electrode rods 191, 141 extend outward from the outer end of the bulb and act as outer lead pins which supply power to the cathode 19 and the anode 14. The arc tube 11 is filled with xenon gas.
In a high pressure discharge lamp 10 with the above described arrangement, as is shown in FIG. 9, the cathode 19 is formed of a cylindrical body 192 and a truncated cone-shaped conical part 194 which is located on an end of the body 192 in one piece with it. Furthermore, the diameter is gradually reduced along the cathode axis L in the tip-shaped area 196 (to the left in FIG. 9) and a round, flat tip surface 193 is formed on its tip.
In such a discharge lamp, in order to obtain stable radiant light over a long time, an arc discharge which forms between the electrodes must be stabilized over a long time. For the cathode 19, thoriated tungsten is used in which an emissive material of thorium dioxide (ThO2) is provided. A carbide layer A of tungsten carbide (W2C) is formed on the surface of the base side region 195, which is a region outside of the tip-shaped area 196. This technology is described in JP-A-HEI 10-283921. FIG. 9 is a top view of the cathode in which the carbide layer A is advantageously shown using a broken line.
In this carbide layer A, during the arc discharge, the oxygen of the thorium dioxide (ThO2) is trapped by the tungsten carbide (W2C) and thorium (Th) is supplied with high efficiency to the tip surface 193 of the cathode 19. This thorium feed phenomenon occurs optimally at a temperature of 1400° C. to 1800° C. of the carbide layer A of tungsten carbide (W2C) and is described by the following chemical formulas (formulas 1 and 2).ThO2+W2C→Th+2W+CO2  (formula 1)ThO2+2W2C→Th+4W+2CO  (formula 2)
The thorium feed phenomenon is further described below. The carbide layer formed on the cathode surface does not penetrate only through the cathode surface, but reaches into the interior of the cathode to a depth of roughly 100 μm from the cathode surface. The reactions described above using formulas 1 and 2, therefore, occur not only on the cathode surface, but also within the cathode. The thorium (Th) which forms within the cathode passes through between the grain boundary of the tungsten and is deposited on the cathode surface, while it passes partially through the grain boundaries of the tungsten, moves simultaneously within the cathode and is deposited on the tip surface 193 of the cathode 19.
As a result, after lamp operation over a long time, thorium (Th) can be reliably supplied with high efficiency to the tip surface 193 of the cathode 19 and stable radiant light is obtained over a long time.
If a carbide layer is applied as far as the tip-side region 196 of the cathode 19, the tip-side region 196 reaches roughly 2900° C., melting the tungsten carbide (W2C). This results in the disadvantages of premature wear of the cathode tip, and thus, shortening of the service life and the danger of blackening of the arc tube resulting in premature reduction of the intensity of the radiant light. Therefore, the tip-shaped region 196 of the cathode 19 is no longer provided with a carbide layer.
The thorium feed phenomenon also optimally occurs at a temperature of the carbide layer of 1400° C. to 1800° C. of the carbide layer. A cathode has been suggested in which, as shown in FIG. 10, on the cathode surface, a surface 197 perpendicular to the axis L is formed and is irradiated with light from the arc so that the carbide layer applied to the cathode reliably reaches 1400° C. to 1800° C., and in which, thus, the carbide layer A which has formed on the vertical surface 197 is reliably fixed at the range from 1400° C. to 1800° C. so that the thorium feed phenomenon can be carried out even more flexibly. This technology is described in Japanese Application Publication JP-A 2005-142071 (U.S. Patent Application Publication 2005/0099121 A1). FIG. 10 is a top view of the cathode in which the carbide layer A is advantageously shown using broken lines.
Recently, however, in the field of projector apparatus using DLP® technology, a discharge lamp with an increased amount of xenon gas added, an increased operating pressure of the lamp and a simultaneously reduced distance between the electrodes has been developed, since there is more and more a demand for a point light source lamp with high radiance.
However, in this discharge lamp, there is a tendency for the cathode temperature to increase due to the effect of the radiant heat of the anode, since the anode temperature is higher than in the past. As a result, the thorium (Th) contained in the cathode is prematurely reduced and dried out in a short time. Furthermore, since the tip area of the cathode is shifted into a high-temperature state, the crystal grain size of the tip area of the cathode increases. The movement of the thorium (Th) in the cathode is prevented by the crystal grains. The thorium (Th) is no longer supplied to the cathode tip.
This means that thorium (Th) can no longer be supplied to the cathode tip after lamp operation, resulting in the disadvantages that the flicker phenomenon occurs and images on the screen flicker.